How do you defend against artillery barrages?

buy tanks

Many many tanks

like a big ass herd o' tanks

1m tanks is quite sufficient.

If your offender has 2m tanks he will still walk all over you with ABs if you have 1m tanks. 10k can be sufficent, 1m can be insufficient all depending on who wants to turn you into a parking lot and how badly.

mostly, get more tanks, but

weapons tech boosts your tanks when defending against AB - attacker must use more tanks, thus they use more oil and they suffermore tank casulties

if it is to be a long running exchange toward mutually assured parking lots, high medical tech will help tip the balance in your favor

more cs will allow you take cs losses from the ABs and still rebuild at an effective rate - otherwise you can become trapped till end of set with no income because you have few buildings and even if you have cash, unable to rebuild because you don't have enough turns at your tiny build rate to rebuild before your opponent comes round to blast you again

yea, simply can't have enough defense to be unbreakable.

High weapons tech and medical tech will alleviate your losses in battle. If you're tech is high enough and your military plenty enough, you can make it extremely costly for your enemy to persist in those attacks, and maybe even force him to weaken his other military, thus leaving you an opening to cripple or destroy him.